Check HubConnection state before running invoke logic

Author: mikaelm12

src/SignalR/clients/java/signalr/src/main/java/com/microsoft/signalr/HubConnection.java

@@ -472,6 +472,10 @@ public void send(String method, Object... args) {
      */
     @SuppressWarnings("unchecked")
     public <T> Single<T> invoke(Class<T> returnType, String method, Object... args) {
+        if (hubConnectionState != HubConnectionState.CONNECTED) {
+            throw new RuntimeException("The 'invoke' method cannot be called if the connection is not active");
+        }
+
         String id = connectionState.getNextInvocationId();
         InvocationMessage invocationMessage = new InvocationMessage(id, method, args);
 

src/SignalR/clients/java/signalr/src/test/java/com/microsoft/signalr/HubConnectionTest.java

@@ -903,6 +903,15 @@ public void cannotSendBeforeStart()  {
         assertEquals("The 'send' method cannot be called if the connection is not active", exception.getMessage());
     }
 
+    @Test
+    public void cannotInvokeBeforeStart()  {
+        HubConnection hubConnection = TestUtils.createHubConnection("http://example.com");
+        assertEquals(HubConnectionState.DISCONNECTED, hubConnection.getConnectionState());
+
+        Throwable exception = assertThrows(RuntimeException.class, () -> hubConnection.invoke(String.class, "inc", "arg1"));
+        assertEquals("The 'invoke' method cannot be called if the connection is not active", exception.getMessage());
+    }
+
     @Test
     public void doesNotErrorWhenReceivingInvokeWithIncorrectArgumentLength()  {
         MockTransport mockTransport = new MockTransport();
@@ -1204,4 +1213,4 @@ public void non200FromNegotiateThrowsError() {
             () -> hubConnection.start().timeout(1, TimeUnit.SECONDS).blockingAwait());
         assertEquals("Unexpected status code returned from negotiate: 500 Internal server error.", exception.getMessage());
     }
-}
+}